300B grid resistance - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 19th October 2008, 08:56 PM   #1
cetoole is offline cetoole  United States
diyAudio Member
 
Join Date: Mar 2005
Location: Maryland, USA
Default 300B grid resistance

I have been working on a 6688-300B SE schematic with input from user dsavitsk here, and am at the point where I am happy with it conceptually, but I have one question. The plan is to run fixed bias on the 300B, and reading the WE datasheet, it states that the max grid resistance for fixed bias is 50kohm. I will be using most likely EH 300B's, and the grid bias scheme isnt exactly normal, so I have no idea if this is going to be a problem.
Attached Images
File Type: jpg bias.jpg (42.4 KB, 283 views)
  Reply With Quote
Old 19th October 2008, 09:03 PM   #2
diyAudio Member
 
analog_sa's Avatar
 
Join Date: Aug 2002
Location: Sofia
It will be a problem. Any reason to insist on such a high value?
  Reply With Quote
Old 20th October 2008, 03:22 AM   #3
cetoole is offline cetoole  United States
diyAudio Member
 
Join Date: Mar 2005
Location: Maryland, USA
Mainly to keep the cap size down, and give the driver a bit easier of a load. I have seen plenty of other schematics violating this, apparently without issue, so didn't know if it was all that important. Since it seems to be, I will change it. Make the CCS adjustable from 2mA down, and have a fixed 50k from the grid to ground.
Attached Images
File Type: jpg bias2.jpg (34.8 KB, 213 views)
  Reply With Quote
Old 20th October 2008, 04:09 AM   #4
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Quote:
Originally posted by cetoole
Mainly to keep the cap size down, and give the driver a bit easier of a load. I have seen plenty of other schematics violating this, apparently without issue, so didn't know if it was all that important. Since it seems to be, I will change it. Make the CCS adjustable from 2mA down, and have a fixed 50k from the grid to ground.

I've been designing and building 300B based SE and PP amplifiers for about 14yrs now, and I have not had problems with grid resistances of up to 100K ohms.. Anything significantly higher than that with fixed bias really is asking for trouble. (I rarely use cathode bias in my output stage designs.) Typical 300B draw several or more uA of grid current depending on plate voltage and it can get considerably higher than this before the tube is considered "bad." I've seen north of 10uA in some and even with 100K this did not cause an issue, with 1M it definitely would. AND don't count on it being constant over the life of the tube!

You also comment on making life easier for the driver, but you probably are not thinking about the miller capacitance of this tube and you should. The CCS in the bias circuit is drawn the wrong way around, and had better be adjustable.. Also this scheme can only used with regulated B+ unless you like having your operating point wondering all over the place with changes in your line voltage.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 20th October 2008, 05:04 AM   #5
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
Kevin has it right.

I did some work on an Audio Space Push Pull 300B last week. This amp has fixed biased JJ 300Bs in it. It had 220K grid 1 resistors which I thought was a bit high so I measured the voltage drop across the 220K, exactly 2.0 Volts.
That means 9 uA grid current at idle.

Follow Kevins advice and do not use any larger than 100K.

Cheers,
Ian
  Reply With Quote
Old 20th October 2008, 07:41 AM   #6
diyAudio Member
 
analog_sa's Avatar
 
Join Date: Aug 2002
Location: Sofia
And if the driver has problems driving 50k it will certainly not sound good with the 300B.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
300B SE cathode feedback to grid? alan_982002 Tubes / Valves 3 29th March 2010 01:04 PM
Max. grid resistance stoo Tubes / Valves 7 5th August 2008 11:56 PM
300b LTspice model with grid current? rdf Tubes / Valves 0 24th February 2008 07:08 PM
Control grid Voltage / grid stoppers... jon_010101 Tubes / Valves 2 4th May 2006 01:59 AM
What is the right current for a 300B at 400V and -80V grid bias? jogas Tubes / Valves 11 16th August 2005 06:20 AM


New To Site? Need Help?

All times are GMT. The time now is 07:46 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2